The Autoionization of Water
Water is actually a very unusual substance in many ways. Among these ways is the ability of a water molecule to react with another water molecule. We say that water is "amphoteric", meaning that it can act as both an acid and a base. Notice, I wrote "an acid and a base": I specified 'and' because while some water molecules are behaving as an acid another portion of water molecules are acting as a base at the same time. The process goes on continually because the autoionization of water is an equilibrium reaction. It turns out (very fortunately for us) that water is very, very stable and so very resistant to chemical change. Consequently, only a very tiny fraction of water molecules will be ionized. The total number of ionized water molecules is a fixed constant (at a fixed temperature); what does change is the ratio of water molecules receiving a proton to water molecules donating a proton.
The Water Ionization Product Constant
The constant for the autoionization of water is 1.00 x 10exp-14. As this is an exceedingly small number, most water exists in the intact molecular form. The ionization product constant for water is:
Kw = [H3O+][OH-] = 1.00 x 10exp-14.
The derivation of the ionization product constant for water is explained in the figure below.
As can be seen in the diagram, water ionizes only very slightly. Additionally, any changes in the "massive" molarity of pure water will be tiny compared to the relative change of hydronium and hydroxide ions; upon the addition of an acid or base. Therefore, the water concentration is not expressed in the constant equation and instead contributes toward the value of the water ionization constant.
That's all for now, loyal reader.
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